The mediator pattern defines an object that encapsulates how a set of objects interact. This pattern is considered to be a behavioral pattern due to the way it can alter the program's running behavior.

Like the Façade pattern, I can absolutely see the logic of wanting to use a mediator. It is supposed to make it easier to work with a set of objects, because it hides their interactions.

In practice, almost all known cases are bad ones. In fact, in most systems that I have seen any association of the name to the actual pattern it is supposed to represent is not very associated at all.

The differences between façade and mediator are minute, and you would think the same advice would apply. However, while you can find a lot of usages of facades (or at least things people would call facades), there are very few real world examples of mediator pattern in use. And almost all of them carry the marks that say: “Just read GoF book, @w$0m3!!!”

Isn't a service bus, or any kind of messaging system, a mediator? I'd also consider the events and callbacks parts of jQuery an example of the mediator pattern. I've found both to be very useful.
I've also written small classes hook two services together so I can test that they're wired together correctly. Isn't that a mediator?

Yep, the MessageBus/PubSub is a mediator, and it's commonly found in MVVM frameworks to communicate between view models. It's also used in web apps to communicate between components in a page.

I use Knockout.PostBox (https://github.com/rniemeyer/knockout-postbox) to do pub/sub in my web apps. Very slick. Components (e.g. observables) communicate by publishing messages on the mediator object, ko.postbox.

I even use it for application commands. So my Song object can simply call ko.postbox.publish("Play", this) to tell my application to start playing that song. It doesn't care who is listening for that command, thus decoupling the implementation from the usage. Works out nicely.

Euproric,
Think about the distinction between Facade & Mediator, or Stategy & State.
Those are really subtle distinctions, but they need different names, to express their differences.
To call MessageBus a Mediator is the same, except the gap is FAR larger.